(a) Field of the Invention
This invention relates to the removal of fluoride from water and more particularly relates to its removal as a water insoluble precipitate.
(b) History of the Prior Art
Many industrial processes including those processes for the manufacture of phosphates, fluoborates, fluorides, fluorine, and fluorosilicate have large quantities of waste water which contain fluoride. For the protection of the enviroment, it is necessary to remove fluoride ion from the waste water prior to its discharge. Governmental regulations presently provide for limitations on the discharge of fluorides to the environment and generally removal of as much fluoride as possible prior to its discharge. Additionally, it is sometimes desirable to remove naturally occurring fluoride from drinking water.
In the prior art, methods have been developed for removing fluorides from water. None of these processes for removing fluorides have been completely successful since even the best of the commercially feasible processes result in the water continuing to contain at least about four parts per million and usually in excess of ten parts per million of fluoride.
Generally, the prior art processes precipitate fluoride contained in the water as calcium fluoride. Such precipitation usually did not result in the removal of sufficient fluoride to drop the fluoride concentration in the water below eight parts per million. When, subsequent to initial removal of calcium fluoride, large quantities of calcium compounds were added to the water, for example in the form of lime, additional precipitate would form which would drop the fluoride concentration to as low as about four parts per million if long holding times were used.
No commercially feasible prior art process, however removed sufficient fluoride from the water to drop the fluoride concentration to below about three parts per million. A lower concentration of fluoride in discharged water is desirable for better protection of the environment.
The ocean has been able over long periods of time to free itself from many soluble fluorides and presently contains about 1.4 parts per million of soluble fluoride. The ocean has been able to remove fluorides in the form of water insoluble calcium salts, some of which have converted to highly insoluble fluorapatite. However, since three moles of phosphate are required for each mole of fluoride to form fluorpatite and since the ocean contains less than about 0.1 parts per million water soluble phosphate, the ocean is unable to remove the remaining soluble fluorides as fluorapatite.